1. Field of the Invention
The present invention relates to a sheet attachment detecting apparatus that is capable of detecting a phenomenon that a sheet is attached around a rotary member that conveys the sheet when the phenomenon occurs, and a fixing apparatus and an image forming apparatus each having the sheet attachment detecting apparatus.
2. Related Background Art
At present, many image forming apparatuses such as printers or copying machines are electrophotographic systems. In this system, a toner image formed on the photosensitive drum is transferred onto a sheet conveyed at a transfer position to form an image. However, because the toner image is unstable after the toner image is transferred onto the sheet, the sheet is conveyed to a fixing apparatus where the toner image is fixed onto the sheet.
The fixing apparatus which is made up of a pair of rollers consisting of a fixing roller having a heater therein and a pressure roller that presses the sheet under a predetermined pressure to the fixing roller is widely employed because it is simple in structure.
That is, while the sheet is nipped between and conveyed by both of the rollers that are rotating, heat and pressure are applied to the sheet, to thereby permit the toner image to be permanently fixed around the sheet.
However, there is a case in which the sheet is attached onto the surface of the rollers while the sheet is nipped and conveyed by the pair of rollers. Up to now, even if the sheet is attached around the fixing roller or the pressure roller, there was provided no means for detecting that fact directly.
For that reason, when a power supply of the apparatus turns on and off in a state where the sheet is attached around the roller, the roller is driven and rotated, and the sheet is further attached around the roller, as a result of which there may occur severe sheet jamming.
This phenomenon is not limited to the rollers in the fixing apparatus, but may occur in any roller in the case where the sheet is conveyed by the rollers.
The present invention has been made to solve the above problem, and therefore an object of the present invention is to provide a sheet attachment detecting apparatus that is capable of detecting a phenomenon that a sheet is attached around (wound round) a rotary member that rotates to convey the sheet when the phenomenon occurs, and a fixing apparatus and an image forming apparatus each having the sheet attachment detecting apparatus.
In order to achieve the above object, according to the present invention, there is provided a representative structure comprising: a rotary member which is rotatable for conveying a sheet and whose surface is electrically conductive; an electrically conductive member which can be abutted on and separated from the surface of the rotary member; and an attachment detecting means which applies a power to the rotary member and the electrically conductive member for detecting the attachment of the sheet around the rotary member.
In the above structure, when the sheet is attached around the rotary member, the sheet exists between the electrically conductive member and the rotary member. As a result, a potential difference is different between a case where a sheet does not exist between the electrically conductive member and the rotary member and a case where a sheet exists therebetween when the power is applied to the electrically conductive member. Accordingly, the attachment of the sheet around the rotary member is detected by detecting a voltage across the electrically conductive member.
Because the present invention is structured as described above, the attachment of the sheet around the rotary member can be detected. For that reason, the severe jamming can be prevented in advance by stopping the drive of the apparatus or the like at the time when the attachment of the sheet around the rotary member is detected.